Detecting and locating fluid leaks beneath slab-on-grade foundations is often a difficult and destructive task. Because the slab blocks access to the soil underneath, one must either break up the slab to inspect the subjacent soil or drill relatively large holes through the slab to bore soil samples. Further, these methods require a certain amount of guesswork because of the uncertain location of suspected leaks. Consequently, a number of holes must be drilled or even jack hammered through the slab before the actual leak is located.
A less destructive method of detecting and locating leaks beneath slab foundations involves directing a radar signal down through the slab and into the subjacent soil. Because the radar return from wet soil will differ from that of dry soil, using radar may allow one to approximate a leak location without partially destroying the slab. However, certain types of reinforced slab foundations may attenuate the radar signal sufficiently such that an accurate return from the subjacent soil is unobtainable. Moreover, radar is not well suited to detecting small leaks, nor for differentiating between small leaks and variations in soil composition or the presence of roots and other plant matter.
Methods currently exist for detecting and locating leaks from landfills, hazardous waste dumps, impoundments, and other outdoor fluid containment areas by measuring changes in the conductivity and/or resistivity of the adjacent soil. Daily et al. '406 discloses mise-a-la-masse and electrical resistance tomography leak location methods. Mise-a-la-masse involves driving an electrode within a fluid containment facility to an electrical potential with respect to another electrode placed at a distance from the facility. Voltage differences are then measured between various combinations of additional electrodes placed in the soil adjacent to the facility. The leak location is located by determining the coordinates of a current source pole that best fits the measured potentials within the constraints of the known or assumed resistivity distribution. Because the potentially leaking fluid must be driven to a potential, mise-a-la-masse methods can monitor for leaks in continuous fluid systems only, such as ponds, lined fluid containment areas, and tanks.
Electrical resistivity tomography (ERT) involves placing electrodes around the periphery of, beneath, or, in the case of subsurface containment vessels, above the facility. A known current is applied to alternating pairs of electrodes, then the electrical potential is measured across other alternating pairs of electrodes. From this data the electrical resistivity at a plurality of points in the soil can be calculated. Disturbances in resistivity will correlate with migration of leaking fluid. However, Daily does not disclose a method or apparatus that allows the electrodes to be placed directly under the leak source, after construction of a building or other structure.
Henderson '202 and '045 both disclose directly monitoring the soil subjacent to a fluid containment area by burying electrodes directly beneath the containment. Both Henderson patents disclose a plurality of four-plate electrode systems. A voltage and a known current are applied across the outer pair of plates. The resulting potential difference is measured across the inner pair. Henderson '045 also discloses a system of individual electrodes that, by varying the spacing between the electrodes impressing a current into the ground and the spacing of the potential measurement electrodes, can indirectly measure the resistivity at a calculated depth. However, Henderson '045 does not disclose a method of directly monitoring the subgrade beneath a structure without permanently burying the electrodes or a method to place electrodes beneath an existing structure.
Woods et. al '244 discloses a leak detection system for radioactive waste storage tanks. The system comprises a metal tank, an AC generator connected between the tank and a reference electrode, and a plurality of reference electrodes. When the generator is energized, it creates an electric field in the ground between the tank and the reference electrode. A voltmeter measures the potential difference between the sensing electrodes and the tank. A significant change in the potential at one or more of the sensing electrodes indicates that the tank has developed a leak. Woods et al. has a number of disadvantages. First, it requires an electrically conductive fluid container. Second, it requires that the electrodes be permanently buried in the soil surrounding the tank. Finally, it requires the use of an AC generator, which is less convenient than a DC power source.
Bryant '625 discloses a method and apparatus for creating an electrical resistivity map of the volume beneath a slab foundation or other structure by placing electrodes through a foundation, and applying a current through them. Bryant '625 further discloses a method for converting the measured potential to a resistivity value, assigning the resistivity value to a spatial coordinate, and storing a plurality of values in computer files. The apparatus includes an array of electrodes that are used to impress a known current in the soil and measure the resulting electrical potential of electrodes. Typically, a pair of electrodes are used to impress a constant current, and another pair are used to measure a voltage potential.
The array of electrodes are interconnected by electrical conducting cables that connect to the various electrodes at predetermined intervals. The interconnecting cables transmit electrical current that passes though certain electrodes to create the electrical field within the underlying and subjacent soil, and return electrical signals from other measuring electrodes that detect the electrical field within the soil. However, Bryant does not disclose the ability to switch current between nodes or to conduct an orderly permutation of voltage measurements between nodes.
None of the prior art is entirely satisfactory to locate fluid leaks beneath an existing slab foundation and analyze them in near real time. For instance, it is not practical to electrify the potentially leaking fluid—typically a plumbing system—and because there may exist multiple sources of fluid, mise-a-la-masse is not a practical option. Nor is it practical to embed permanently a series of electrodes beneath the slab to monitor soil resistivity. Further, because some of the ERT methods—for example, the Henderson references—use multiple-plate electrodes, a large hole would have to be bored into the slab to insert the electrodes into the subjacent soil making the method impractical and destructive. In addition, placing the electrodes around the periphery of the foundation is less accurate compared to placing the electrodes directly beneath the potential leak source.
Some methods of employing wireless sensors are available in the art. For example, those shown in U.S. Pat. No. 6,332,109 B1 to Sheard. Sheard discloses a data acquisition system for obtaining data from a geological surface at a plurality of equally spaced apart locations using nodes. FIG. 1 of that patent shows nodes which can wirelessly transmit information such as a characteristic voltage to a receiver. However, Sheard does not provide a method of distributing current to various nodes for systematically reading all the permutations of voltages provided pursuant to the distributed current to provide a complete and reliable data set. Also, Sheard does not provide a method to address each node dynamically so that the nodes are truably interchangeable.
Another example is U.S. Pat. No. 6,226,601 B1 to Longaker. Longaker provides a wireless seismic survey system that is structured as a hierarchy of cell network layers, the higher network layers covering a larger area and the lower network layers covering smaller areas. However, Longaker does not provide for multiple positions of injection current or the controlled monitoring of a permutation of voltages between nodes. Also, Longaker does not provide for dynamically addressable sensors to allow for easy interchangeability.
The current state of the art is unsatisfactory because it does not provide a method to remotely change injection current locations or to conduct an orderly progression through a permutation of voltages and currents between nodes. Moreover, the state of the art does not provide for dynamically addressable sensors whose location and address can be changed on the fly.
Furthermore, the present state of the art requires that the electrodes be placed in a linear, regularly spaced grid pattern that does not provide needed flexibility in the physical layout of arrays of electrodes in multiple, non-linear arrangements. The present state of the art presumes that the locations of the electrodes to one another. It may be necessary to arrange the electrodes in a non-linear grid if the physical layout of the foundation or underlying area to be measured prevents the layout of electrodes in a typical, linear grid arrangement. It may be necessary to locate certain electrodes in a non-linear pattern to accommodate obstructions or to conform the layout of the grid to a particular slab configuration or other geographic anomaly. Where a need arises to arrange the electrodes in a non-linear grid, it may be necessary to identify the spatial relationship of the electrodes. Likewise, it may be necessary to adjust the location of certain electrodes to accommodate obstructions and thus, it may be necessary to identify the new location.
Furthermore, the present state of the art requires that electrodes be placed according to a measured or surveyed pattern at the physical location. The requirement of physically measuring and placing electrodes is hampered by structures which include walls or other obstructions which makes placing the electrodes accurately difficult. The inaccurate placement of electrodes leads to errors in the mathematical calculations required to analyze the locations of the anomalies and therefore reduces the overall efficiency and accuracy of the system.
The present state of the art is further unsatisfactory because the sensors are affixed to the interconnecting cables and to the electrodes at predetermined intervals. A sensor or a part of the interconnecting cable or an electrode may become damaged or cease to function and require replacement. Physical damage to or failure of a given sensor within a long strand of conducting cable requires replacement of the sensor or substitution of a conducting cable, frequently associated with a substantial downtime for repair or replacement. In order to repair a part of the interconnecting cable, sensor and electrode combination, if may be necessary to remove the entire section of cable with a plurality of sensors and electrodes, from service and effect expensive and time-consuming repairs.
Moreover, the state of the art does not provide for interchangeability of sensors. The uniqueness of sensors required by the prior art creates a need for specific sensors uniquely identified by their order in a specific grid. This limitation requires extensive time in setting up a grid and replacing defective sensors.
Furthermore, the sensors are typically arranged on long sections of interconnecting conductive cabling used to transmit control signals, measurement signals and impressing current. The voltage loses in such cabling are substantial and require corrective and potentially inaccurate algorithms in the analytical software used to generate the subterranean mapping.
Accordingly, a need exists for a method for locating fluid leaks and other anomalies beneath slab-on-grade foundations without the need for destroying the slab or permanently burying electrodes beneath the slab. Additionally, a need exists for a method and apparatus to map a three-dimensional subjacent volume's electrical resistivity with both static and dynamic electrodes. A further need exists for a method to accurately place electrodes in a grid.
A need further exists in the art for a method of wirelessly reconfiguring the position of current sources and voltage meters among electrodes. A need further exists for a non-unique sensor attached to the electrodes which is addressable and reconfigurable so that expensive and time consuming cable and sensor unit replacement is unnecessary. A need further exists for real time three-dimensional movies of anomaly movements which are subsurface. Further, a need exists for accurate measurement of voltage offsets to accurately calibrate voltage readings among sensors.